1. Field of the Invention
The present invention relates to non-destructive metal inspection apparatus and more particularly to a pin coil-shaped detector adapted to be used in magnetized non-destructive type metal inspection apparatus.
2. Description of the Prior Art
Conventionally, various kinds of non-destructive type metal inspection methods and apparatus are known. These can be classified as the X-ray inspection method, ultrasonic flaw detecting method and a magnetic flaw detecting method. The magnetic flaw detecting method is divided into the magnetized powder inspection method using magnetized iron powder and the eddy current inspection method for inspecting a specimen to be tested by inducing an alternating current magnetic field in the specimen. The eddy current type metallic flaw inspection apparatus of the prior art requires a strong and large-size magnetizing apparatus so that it is laborious and uneconomical to produce the flaw inspection apparatus and perform the inspection. Further, the specimen to be inspected is limited to products made of nonmagnetic material. Moreover, the inspected specimen, stress-strain and magnetism diffusion along the surface are apt to be generated.
Therefore, if the eddy current type metallic flaw inspecting method and the apparatus therefor are applied to a ferromagnetic material, such as a steel plate or a steel tube, the material must be fully magnetized in order to inspect the products effectively and precisely.
That is to say, in order to cause an alternating magnetic field to penetrate into an inner portion of the ferromagnetic material to be inspected, it is necessary to fully magnetize a disturbed magnetic field in the material or effectively arrange the molecules in order in the material by passing a strong direct current magnetic field through the material to be inspected.